Certain aspects of the present disclosure generally relate to the field of geophysical surveying and may have particular applicability to electromagnetic (EM) surveying in marine or other settings.
In geophysical prospecting in a marine environment, sources and receivers are used to understand the geography of the earth below the water. In a particular surveying method, sources of electromagnetic radiation or fields are deployed according to a desired arrangement to direct electromagnetic radiation toward the earth. The radiation interacts with structures and materials in the earth, and the interaction changes the radiation according to properties of the structures and materials. The changed radiation is detected by receivers, which record data representing the changed radiation. The data is then analyzed to understand the properties of the earth. The electromagnetic radiation may take the form of a wavefield that propagates through the water and into the earth. The changed radiation also typically propagates as a wavefield. The sources and receivers may be stand-alone devices, or may be arranged in elongated assemblies. The assemblies may be towed behind a vessel, or may be stationary in the water or on the sea floor. In some cases, receiver assemblies are referred to as “streamers”.
In order to understand properties of the materials and structures in the earth, a model is typically used to derive the properties from the recorded data. In the case of electromagnetic surveying, the recorded data are typically voltages, and these voltages, related to characteristics of the source radiation and the geometry of the source and receiver arrangement, indicate the transformation of the radiation by the structures and materials in the earth. The transformation, in turn, indicates physical properties of the materials such as resistivity, magnetic permeability, density, and other physical properties. Using a physical model that relates such physical properties to transformations in electromagnetic radiation, the physical parameters can be iteratively determined by computing results from the model based on a representation of the known source radiation, the geometry of the survey, and estimates of the physical properties. Agreement of the model results with the detected radiation indicates the accuracy of the estimate, and if such accuracy is inadequate, the estimate is refined until a desired accuracy is reached. This process is typically called inversion. The sub-process of calculating model results, as part of the inversion process, is typically referred to as “forward modeling”.
In a typical geophysical surveying process employing inversion, a large amount of data is collected over a wide geographical area. For purposes of inverting the data, the volume under the surveyed geographical area is represented as a “grid” or matrix of “cells”, each representing a small volume of the surveyed area. Each cell may be modeled as having uniform physical properties, so that one value of a physical property applies to the entire cell. The physical property is estimated, and the forward model is computed to give an estimate of the recorded data. The estimate is compared to the recorded data, and the quality of the fit is judged and compared to one or more criteria. If the quality of the fit is insufficient, the estimate of the physical property of each cell is adjusted, and the forward model recomputed. This iterative process continues until the fit criteria are met, at which point the inversion is said to have “converged”.
Various methods of converging inversion problems are typically used. The estimates may be refined using various types of Monte Carlo methods, linearized neighborhood approximations, Gauss-Newton search methods, and the like. Such methods typically require vast computing resources, and even then may be unacceptably slow.
Therefore, techniques for geophysical inversion that reduce computational time and resource requirements are needed.
To facilitate understanding, identical reference numerals have been used, where possible, to designate identical elements that are common to the figures. It is contemplated that elements disclosed in one aspect may be beneficially utilized on other aspects without specific recitation.